The invention relates to a method and to an apparatus for determining at least one physical parameter of accumulator electrode plates, such as the true plate thickness, the apparent density, the true density, the pore volume and/or the porosity, and/or the pore structure.
Just as in conventional lead sulfuric acid accumulators, positive and negative electrode plates are used in fleece-bound, recombinable lead sulfuric acid accumulators which substantially comprise a metal grid, which serves as a material carrier and for carrying away the electrical current, and the positive or negative active material. In order to achieve high capacities and/or high discharge currents, a large contact surface between the active material, which is made of lead dioxide on the positive side and lead on the negative side, and the sulfuric acid serving as the electrolyte, is necessary, since the generally known electrochemical reactions in lead sulfuric acid accumulators takes place substantially at these contact surfaces. For this reason, the active materials must have a porous structure.
Furthermore, it must be ensured that during discharge of the accumulator the sulfuric acid itself takes part in the electrochemical reactions which are taking place so that it is used up. Consequently, the hollow volumes in the active materials are also of import as storage space for the quantity of sulfuric acid required during a discharge.
Moreover, the lead sulfate which forms both on the positive and the negative plates during discharge has a larger volume than the end products so that a reduction of the pore volume occurs during discharge. This is highly disadvantageous for the transport of further sulfuric acid out of the space between the electrode plates into the electrode plates. For these reasons, the active materials must have a suitable porous structure, i.e. the proportion of hollow volume to entire volume of the active material must attain a particular value, or the apparent density and the true density of the active material must have a particular ratio, in order to allow all the transport processes coupled with the electrochemical reaction to proceed at the required speed.
Apparent density is understood to mean the quotient of the weight of the material and its total volume including the porous volume. The true density of the material is obtained by dividing its weight by the total volume less the pore volume.
It is thus of major importance during the manufacture of the active material from positive and negative accumulator electrode plates to produce an optimum porous structure. By optimum structure it is meant that the proportion of hollow volume in the active material should indeed be relatively large but not too large since otherwise the lifetime of the electrode plates is reduced.
In recombination batteries comprising glass-fiber separators, the acid fill level of the separators assumes a key importance for the performance of the accumulator. Consequently, the plate thicknesses must correspond exactly to the design values in order to maintain the set pressing power and the free pore volume of the separator constant. For this, it is particularly important to determine the true plate thickness, i.e. the thickness which is obtained from the true volume taking into to account all the uneven features of the plates such as, for example surface indentations or fissures.
There are a large number of parameters used in the course of the manufacturing process which later affect the structure of the active material and also the plate thickness. The most important are the recipe as well as the conditions during the manufacture of the moist pastes and during its incorporation into the grid material, and also the parameters used during maturing, drying and formation of the plates.
Numerous methods are known for determining the plate thickness. A very simple method consists of a manual measurement with Vernier callipers or with some other mechanical thickness measuring device.
It is also already known (DE 38 26 516 A1) to use a laser thickness measurement device for determining the thickness of freshly pasted electrode plates, although this involves a high level of technical complexity.
All these known methods have the common disadvantage that the measurement result obtained for the plate thickness is not the true plate thickness, but is rather a too large a plate thickness, this being particularly important for recombination batteries. The reason for this is the rough and non-uniform surface of the electrode plates which plays a primary roll in raising the surface during the thickness measurement.
The conventional measurement methods also do not allow anything to be ascertained about the structure of the active material.
As a result of the complexity and imprecision of the known measurement methods, investigations of the porous structure of the active material for monitoring the production steps are not carried out during the manufacture of the electrode plates. That is to say, the known measurement methods are not employable as production controls or as dimensions for production control.
Methods for characterisation of porous solid bodies with regard to the proportion of hollow volume in the entire body and the structure of the hollow volumes themselves, such as mercury porosimetry or porosity measurements are know from the following literature: Drotschmann, C. Bleiakkumulatoren, Weinheim 1951, page 132; Drotschmann, C. Batterien 11 (1943), page 207; Manegold, E. Kolloid-Zeitschrift 81 (1937); Samsonov, P. D. Kuznezova, N. G., Journal angewandten Chemie 4 (1941), page 318.
These relate exclusively to laboratory methods which are inherently unsuited to monitoring of the production since they are slow and require complicated apparatus, and, furthermore, the deployment of qualified personnel.
In mercury porosimetry, the penetration of a non-moistening liquid in a porous body is measured in dependence on the liquid pressure, and from this the size and distribution of the pore volumes is deduced. These methods are of considerable complexity as they use high pressure apparatus. Moreover, the investigations can only be carried out by specially trained personnel and are not straightforwardly applicable to negative active materials. The displacement methods in which one measures the amount of liquid which has penetrated into the porous sample body and determines the volume of the entire body which is filled with liquid via a bouyancy measurement are also very complicated and, furthermore, the analysis time amounts to at least 24 hours.
Moreover, the methods known hitherto have the disadvantage of using measurement liquids which have different physical properties, for instance a different surface tension than sulfuric acid, and thus occupy different pore spaces than sulfuric acid.